Fixing up a Marshall 6101

[mayfly]

So, a buddy of mine, Let's call him Bill, asked me to fix up his old Marshall 6101 combo so he can sell it.

He's a gigging guitarist, probably does a couple of shows a month.  He had used the amp for years, but then got into fenders, and then got one of my amps.  He wasn't using the Marshall, so he loaned it to his son.  When he lent it to him, it was in excellent condition.  Sadly, the son was, er, not too amp savvy and when the amp was returned it was sporting mis-matched tubes, broken knobs, and general poor tone. 

I've got some years experience in this kind of thing, so I agreed to put a new set of tubes in it, set the bias, and clean it up for him.

I first played through the amp for about an hour to see how it behaved.  Seemed to work fine, so it was time for a visual inspection and disassembly.

Here's the front, complete with broken knobs:

Here's the backside (power tubes were removed)

This is when I noticed the first strange-ass thing.  The tubes in it were 6L6,  pair of sovteks, and a (mismatched) pair of winged C.  Note the heat damage to the bases of the wingCs:

Turns out that at the time this amp was shipping (early 1990s), Marshall could not get a reliable EL34s, so they switched to 5881s.  The 5881 could be viewed as a super rugged 6L6, except that they can withstand much higher plate voltages and dissipation.  Plugging 6L6s into a circuit designed for 5881s is asking for failures, but unfortunately it's common since your local music store likely does not stock any 5881s.  In this amp, Marshall even put a label on the chassis warning punters not to do this.  Sadly, the label was ignored  :doh:

More in a bit as I take this sucker apart

 

[metalman22105]

Awesome! Keep us posted. Psh, I guess the label wasn't big enough? Hah!

 

[blue313]

Cool thread man.

Those Winged C's aren't the cheapest things to just cook in there.  :laughing7:

 

[jimh]

Are we calling him Bill, because that's his name?

Shame that the amp want looked after. I'm sure you can fix 'er up though. It'll be a good little runner when its done.

 

[mayfly]

Update.  The new tubes and replacement knobs are on order.  I used my old standby supplier:  Antique Electronic Supply.  They've got the best selection and prices are a lot lower than the ripoff, er I mean boutique, amp parts suppliers that have sprung up in the last while.

Next - let's have a look at the footswitch...

 

[mayfly]

So, I got a copy of the schematic to help with setting the bias on this puppy.  It's a typical marshall setup - only one bias pot to set the bias on all 4 tubes.  However, the circuit does have separate cathode fuses for each push/pull pair, so at least that was something.

Anyway, I put it on the bench and noted where the bias was set (-42V, as instructed by the schematic).  Just setting the bias to a marked value is not something that I'm a fan of (I either use current sensing resistors or the old 'scope method), but at least this was not touched from stock.  However, this would have made life a bit more difficult for those 6L6 tubes that the amp was fitted with.

Then I noticed that the anode fuse holders were a bit roughed up - like someone tried to get them open with a screwdriver that was too big.  So, I had a look and sure enough, one of the fuses was blown:

.

That's right - one push pull pair was not working at all!  This would have made the amp run as a 50Watt amp instead of a 100W amp.  Bill had mentioned an apparent drop in volume - well this is likely the cause.  It also means that one of those pairs of tubes is definitely junk. 

Finally, on closer inspection it turns out that the fuses were the wrong value - they were 750mA  instead of the rated 500mA.  I suspect that the wrong tubes started to go and draw more current, causing the original fuses to repeatedly blow, and someone just increased the value of the fuses as a stop-gap.  Not that good.

At least the fuses prevented the frying of the O/P transformer  :headbang:

P.S. I'm reading a set of books right now on tube amp design.  The author states that B+ (or by extension cathode) fuses are not really required in tube amps.  This raised my eyebrows at the time since I always included them - but now I have proof that they are a good idea  ccasion14:

 

[Justinginn]

Wow. I think you need to keep the amp just because you're saving its life and will probably fall in love with it by the time you've poured all that effort into fixing it anyway.  :icon_thumright:

 

[mayfly]

Here's the amp out of the box.

This is a three channel do-everything type amp and it is seriously complicated inside.  It's even got an intel microprocessor in there!

amps like this are very very difficult to work on - almost to the point where if something seriously goes wrong with it, it's not repairable.  If you are considering buying one of these multi-knob do-everything amps, please keep that in mind... and keep a spare if you are seriously gigging.

Suddenly that 66 super with a bunch of pedals out front is looking like a great rig.  At least you can fix it if it goes wrong.  ccasion14:

 

[jimh]

I dare you to rip the guts out, save the knobs and transformers and put a nice all tube point to point circuit in there.  Maybe a Ceriatone Overtone special (dumble style) or something.
:blob7:

 

[mayfly]

I dare you to rip the guts out, save the knobs and transformers and put a nice all tube point to point circuit in there.  Maybe a Ceriatone Overtone special (dumble style) or something.
:blob7:

if i did that, half the knobs wouldn't do anything! :icon_biggrin:

 

[blue313]

I dare you to rip the guts out, save the knobs and transformers and put a nice all tube point to point circuit in there.  Maybe a Ceriatone Overtone special (dumble style) or something.
:blob7:

if i did that, half the knobs wouldn't do anything! :icon_biggrin:

Well, if you have some old pedals kicking around....  :laughing7:

 

[Superlizard]

This is precisely what is wrong with the amp biz today... that thing looks like a friggin' gutted laptop.

And ya gotta love those PCB-mounted pots...  :tard:

 

[Mor Paul]

Shouldn't amps be cheaper if they tried making them the old way?
Probably how the Valve Junior was cheap, and same with Blackheart.

 

[mayfly]

Shouldn't amps be cheaper if they tried making them the old way?

not really.  More labour costs in the old way.  PCB's are cheap to build.

 

[mayfly]

with everything on order, there's nothing to do but clean up the amp's enclosure.

For fun, we vacuumed it out, cleaned the tolex with hot soapy water, and tightened up the handle.  The latter had gotten really loose and rattly.
With marshall handles, you pull up on the cover plate to access the screws:

That's all we'll have until the tubes arrive.


edit -

I also cleaned the pots.

 

[mayfly]

Well, the tubes and the knobs arrived.  We have a matched quad of sovtek 5881, and a package of 8 marshall push-on knobs:

Here's the chassis with the tubes and knobs installed:

Now to bias them up.  There are several ways of biasing and amp:  set the bias voltage to a published value (not the best), measure the plate dissipation (probably the best), and the 'scope' method.  Since the plate dissipation method requires some cathode resistors installed (or a bias probe - which I don't have for octal base tubes), I decided to go with the good old 'scope' method.

First, we need a dummy load since we'll be running this sucker at full power.  Sometimes the original is best:

Then fire up your signal generator. I set it to 1.5kHz.  Well, pretty close  :

I then set the bias voltage to the most negative value it would go to.  This biases the tubes 'cold' and gives us a good starting point for our adjustments.

Then hook up your handy two channel scope.  Mine is an ancient beast, but I never felt the urge to replace it.  The top trace is the input signal measured at the input jack, and the bottom trace is the speaker output.  The amp was set to run full power into the dummy load.

Note the 'notches' in the output waveform.  These are regions of operation where both tubes are not conducting current.  This is called crossover distortion.  Now, we slowly increase the bias voltage until the crossover distortion is eliminated:

There is still a tiny bit in this photo, which I tweaked out after taking the shot.

After it was all said and done, here's the resulting bias voltage:

The schematic says to set this to -42 volts, so we are biased on the 'cool' side.  I left it like this since I didn't want to take the chance of the amp blowing a fuse on whoever Bills sells it to. Of course, it has to sound good like this for it to be acceptable.  Fortunately it sounded just fine - full and LOUD.

I put the amp back into the cabinet and ran it through it's paces.  Everything worked fine.  Then I hooked up the footswitch and found that the LED for channel 3 did not light.  The cause of this was a botched repair done in the past:

Here, the traces for the LED had been ripped right off of the board in a previous LED replacement (top left).  To fix, I scraped the remaining traces clean of the green epoxy, tinned the traces, and made the connection with some thin bus wire.  Works great now.

Here it is.  All fixed up and ready to rock!

 

[blue313]

Pretty interesting man.  Thats supposed to be a pretty nice amp.  Does he have the 12" extension cab?

 

[Wana_make_a_guitar]

Thats nice. Send It this way!  :icon_biggrin:

 

[mayfly]

Pretty interesting man.  Thats supposed to be a pretty nice amp.  Does he have the 12" extension cab?

nope.  are you thinking of buying it? :icon_biggrin:

 

[simple]

How much will your buddy be asking for it?